The present invention relates to a heavy-transport radial tire. In particular, the present invention relates to a heavy-transport TBR (Truck and Bus Radial) tire.
In tire manufacturing, a heavy-transport radial tire is known comprising a radial carcass defined by a body ply reinforced with normally steel wires coated with elastomeric material; a tread having two lateral shoulders; and a tread belt interposed between the tread and carcass and defined by a pack of superimposed tread plies reinforced with wires inclined with respect to the radial wires of the body ply and having two opposite outer coating skims of elastomeric material.
Radial tires of the above type also have, at each shoulder, an annular shoulder cushion made of elastomeric material with a relatively low M200% modulusxe2x80x94at any rate lower than the M200% modulus of the normally similar elastomeric materials coating the tread plies and body plyxe2x80x94and which acts as a shock absorber between a relative lateral edge of the tread belt and the body ply, and is inserted for the dual purpose of increasing the wear resistance of the tread and the working life of the carcass. The two annular shoulder cushions, in fact, provide for distributing the internal stress of the carcass (which would otherwise be concentrated at the equatorial region of the tread) over the whole tread surface, so as to maximize the tread print and so minimize the specific forces applied to each point on the tread and the wear produced by such forces. The two annular shoulder cushions also provide for protecting the carcass by dissipating the forces exchanged between the carcassxe2x80x94as this expands under relatively high operating temperatures and stressxe2x80x94and the tread belt, which by nature is substantially inextensible.
Experience has shown that, in radial tires of the above type, relatively high temperatures and pressures under severe working conditions resultxe2x80x94at least at the thinnest portion of each annular shoulder cushion, ie. the portion adjacent to the equatorial plane of the tirexe2x80x94in substantially total compression of the annular shoulder cushions, and in total failure of the cushions to absorb cyclic circumferential expansion of the body ply. Being practically inabsorbable by the tread belt, cyclic circumferential expansion of the body ply results in fatigue shearing stress of the tread belt capable of detaching the coating skims from the relative reinforcing wires, and of generating microfractures, which, originating at the point of detachment, spread through the coating skims and, encountering practically no obstacles along the way, eventually through the annular shoulder cushions until the tread belt eventually comes away from the body ply.
One aspect of the present invention is a heavy-transport TBR tire designed to eliminate the aforementioned drawback.
More specifically, the heavy-transport TBR tire not only minimizes the risk of the reinforcing wires coming away from the coating skims in the tread plies and, hence, microfractures spreading through the coating skims, but also minimizes the risk of any microfractures which might still be formed spreading through the annular shoulder cushions.
According to the present invention, there is provided a heavy-transport radial tire comprising a radial carcass, in turn, comprising a body ply reinforced with radial wires coated with a first elastomeric material; a tread having two lateral shoulders; a tread belt interposed between the tread and the carcass and comprising a pack of superimposed tread plies reinforced with wires inclined with respect to the radial wires of the body ply and coated with a second elastomeric material having an M200% modulus similar to that of said first elastomeric material, each tread ply having two lateral edges, each adjacent to a respective said shoulder; and, at each said shoulder, an annular shoulder cushion of a third elastomeric material of a relatively low M200% modulus with respect to the M200% moduli of said first and said second material; and also comprising a belt cushion gum strip made of a fourth elastomeric material of a relatively high M200% modulus, but ranging between the M200% moduli of said first and said second material, on the one hand, and the M200% modulus of said third material, on the other, and interposed between the relative annular shoulder cushion and the tread belt to cover at least the lateral edges of those of the tread plies facing the annular shoulder cushion.